TRUCK SERVICE MANUALTM 5-4210-230-14&P-1ELECTRICALFig. 4 Schematic Diagram of Primary and Secondary Circuitsof 12-Volt Ignition Systemobtain greatly improved starting performance at lowtemperatures, the resistor is bypassed during cranking,thereby connecting ignition coil directly to battery. Thismakes full battery voltage available to coil and, thus, keepsignition voltage as high as possible during cranking. Thebypassing of ignition resistor during cranking is accomplishedby use of a "finger" within the solenoid which is connected tothe ignition coil. As solenoid contacts close, the ignition coil isconnected to battery through the solenoid "finger". To preventthe engine from firing while making underhood cranking testson trucks using 12-volt system, ground distributor primarylead.COIL POLARITYOne of the major causes for hard starting or spark plugmisfiring under load results when the ignition coil lead wire tothe distributor is installed on the wrong side of the coil. Thiscondition causes reversed coil polarity.Voltage at the spark plug terminals should always benegative. Whether it is or not depends on how the primaryleads are attached to the coil. Remember, primary lead hook-up directly affects coil polarity, which in turn determineswhether voltage at the spark plug terminals is negative orpositive. On IH trucks the distributor wire to the coil shouldalways be placed on the negative side of the coil if theelectrical system is negative grounded, or onthe positive side if the electrical system is positive grounded.If the primary leads are incorrectly attached, the direction ofcurrent flow through the coil is reversed. Consequently, coilpolarity is reversed, resulting in positive voltage beingsupplied to the spark plug terminals.What difference does it make whether positive ornegative voltage is supplied to the spark plug terminals?. Itdirectly affects the amount of voltage required to fire the sparkplugs. When polarity at the spark plug terminals is positive,it's harder for the voltage to jump across the air gap thanwhen polarity at the plug terminal is negative. Just why this isso is related to a pair of electrical theories--the electron theoryand the theory of thermionic emission.According to the electron theory, all current flows fromnegative to positive. The theory of thermionic emission statesessentially it's easier for electrons to leave a hot surface thana cold surface.Combining the two theories, one finds that electronswill always leave a negative charged surface for a positivecharged surface, and they will leave the negatively chargedsurface with more ease when the surface is heated.Spark plug design is such that the center electrodealmost always operates at a higher temperature than theground electrode. Since it's easier for electrons to leave a hotsurface, it is preferred to have the electrons "jump" from thehotter center electrode to the cooler ground electrode.When the center electrode is negatively charged(negative voltage at the spark plug terminals), this is whathappens. Stated another way, putting the negative charge onthe hotter center electrode causes the gap to be ionized atlower voltage. (Ionization is necessary to permit passage ofthe spark through the high resistance of the gases in thecylinder.)When positive voltage is supplied to the plug terminals,which happens when coil polarity is accidentally reversed, thehotter center electrode becomes positive charged.Consequently, electrons must leave the negative chargedground electrode and move to the positive charged centerelectrode. But, since the ground electrode is cooler than thecenter electrode (and remember, it's easier for the electrons toleave a hotter surface), it takes more voltage to make thecurrent jump the gap-in fact, up to 45 percent more. See Fig.5.CTS-2013-E Page 5PRINTED IN UNITED STATES OF AMERICA